As is well known, vehicle theft and vandalism constitute an immense problem, which has been attacked by the proposal of alarm devices responsive to tampering. A relatively simple installation of theft prevention device is that of measuring battery voltage and actuating an alarm upon voltage reduction, such as would be occasioned by operation of a door light or trunk light. However, prior theft prevention devices operative responsive to battery voltage reduction have not been entirely satisfactory, requiring complex and expensive circuitry, difficult and time consuming installation, and presenting certain operating problems, as in the disarming and arming procedures.